Electric field induction and magnetic field induction develop when energized conductors and cables, carrying alternating current (AC), are in the vicinity of de-energized conductors and cables. However, what are electric field induction and magnetic field induction, really?
When an energized overhead transmission line carries AC, electric fields and magnetic fields are generated. The electric and magnetic fields generated by the energized, current-carrying overhead transmission line will induce a charge into any nearby de-energized line or conductive object through what are called capacitive coupling and inductive coupling. The intensity of the electric and magnetic fields generated by the energized overhead transmission line is dependent on the:
• Energized transmission line’s system voltage
• Amount of current flowing in the energized transmission line
• Proximity of the energized transmission line to other de-energized lines and conductive objects
It is often believed that workers are safe working on a de-energized conductor located in the vicinity of an energized transmission line if the de-energized conductor has been temporarily grounded. However, the process of grounding a de-energized conductor may actually increase the hazard to workers if grounding procedures are not applied correctly. The same concern must also be considered when working on underground cables and cabling systems.
Electric Field Induction
All energized AC conductors create an electric field. The electric field is present due to the AC voltage on the energized conductor with or without current flowing in the energized conductor. We measure this electric field strength in volts per foot or v/ft.
Electric field induction occurs anytime two conductive objects – one energized with AC voltage, creating an electric field around the conductive object, and the second, which is normally de-energized – are separated by a dielectric medium such as air. This arrangement of two conductive objects creates a simple capacitor. This process commonly called capacitive coupling induces a capacitive voltage in the de-energized conductive object.
Again, when a de-energized, ungrounded, conductive object – such as an overhead conductor, cable, shield wire, vehicle, tool, equipment or worker’s body – is positioned near the energized AC conductor, the electric field produced by the energized conductor will induce a capacitive voltage through capacitive coupling into the de-energized object by a process called electric field induction. The de-energized object will retain this capacitive voltage or trapped charge until it is bled off by grounding the conductive object or normal discharge over time.
The electric field induction voltage (capacitive voltage) impressed on the de-energized object is related to the:
• Voltage level of the energized AC source
• Physical position of the de-energized object in relation to the energized source
• Distance between the de-energized object and the energized source
• Total area of the de-energized object exposed to the electric field
The voltage induced into a de-energized object from the electric field of an energized conductor can be very high. However, when the de-energized object is grounded, current flows from the de-energized and now grounded object into the earth (ground source). Grounding the de-energized object to a low-resistance ground source will reduce, but not totally eliminate, the induced voltage. Also, the electric field induction voltage increases as you move down the de-energized conductor, away from the grounding point. The induced voltage measured on the de-energized and grounded object will be the current flowing through the grounds into earth times the impedance of the earth (ground source). If the ground source resistance is very low as normally seen in a multigrounded neutral, substation ground mat or static wire, the generated voltage will be low. However, if the ground source has high resistance, as normally seen when a temporary ground rod is used, the generated voltage can be very high. The voltage and currents generated from electric field induction can be deadly to workers if proper grounding methods are not used.
Where Would I Find Electric Field Induction?
• In a de-energized 115 kV overhead line running under (perpendicular to) an energized 345 kV overhead line
• In a de-energized 230 kV overhead line running parallel to an energized 500 kV line for a distance
• On a nongrounded panel van located inside a substation, and near an energized 230 kV bus
• On a qualified electrical worker’s body, working out of an insulated platform near an energized 230 kV substation bus or an energized 230 kV transmission line
Also, a worker’s body may become charged if they are working out of an insulated platform in an electric field from a nearby energized line. The worker’s body is a floating electrode that will accumulate electrical charge from the electric field generated by the energized conductor. This accumulated charge on the body will be discharged when the worker contacts a grounded device or structure. The capacitive voltage buildup on the worker’s body can be controlled or eliminated with the use of various methods such as:
• Insulating gloves, depending on the induced voltage
• Connecting the worker’s body to the de-energized and properly grounded line or device being worked
• Using a conductive suit
• Bonding the work platform to the grounded conductor or device being worked
Magnetic Field Induction
When AC travels down an energized conductor, it creates an electric and magnetic field. The generated magnetic field creates what is called magnetic flux developed from the current flow in the conductor. If a closed loop of wire is placed near the current-carrying conductor, the magnetic flux from the current flow creates an induced electromagnetic force (voltage) on the loop of wire. This voltage, in turn, will create current flow in the loop of wire. This magnetic process involving current flow in the energized, development of flux, creation of voltage and current flow in a loop of wire is known as Faraday’s law of induction. It can also be explained by looking at a simple transformer. A transformer consists of a high-side winding and a low-side winding both wrapped around an iron core. If the high-side winding is energized and current flows, the magnetic flux from the current flow circulates in the core and induces a voltage and current into the low-side winding, creating a transformer.
When an energized AC transmission line carries current, a magnetic field (flux) is developed around the energized AC transmission line. When a second transmission line, in close proximity and paralleling the first energized AC transmission line, is de-energized and grounded at two distant locations, a conductive loop is created. The varying magnetic flux created by the AC current in the energized transmission line creates an induced voltage in the de-energized and multigrounded transmission line. This voltage will create current flow in the multigrounded transmission line (conductive loop) in a process called magnetic field induction.
The voltage and current induced on the de-energized transmission line (conductive loop) is directly related to the:
• Current in the energized AC transmission line
• Distance between the two transmission lines
• Distance the two transmission lines parallel each other
• Transmission line arrangement, including phase configuration
• Conductor, grounding electrode and earth impedance
Where Would I Find Magnetic Field Induction?
Take a 20-mile-long, de-energized, 230 kV overhead transmission line and close both ground switches at the two substation terminals of the line. You have a conductive loop created by the 20-mile-long 230 kV line, the ground switch at one end, the earth between the two ground switches and the ground switch at the other end. Now, place the de-energized line and closed ground switches in parallel, on the same right-of-way, the full 20 miles, with an energized 230 kV overhead line carrying 850 amps of current. Voltage and current flow will develop in the de-energized and grounded line through the process of magnetic field induction. You have built a transformer with the energized line acting as the high-side winding, the de-energized and grounded line acting as the low-side winding, and the air between the two lines acting as the transformer’s core. This arrangement can be lethal to workers who do not understand the effects of magnetic field induction.
Magnetic field induction on transmission lines can be eliminated by not creating the conductive loop with the de-energized conductor. By not closing the two ground switches on the de-energized line and simply grounding the de-energized transmission line once, at the work location, you totally eliminate magnetic field induction. As you can see, magnetic field induction can easily be eliminated by using proper grounding methods at one location.
A conductive loop is again developed when a worker contacts a de-energized transmission line immersed in magnetic field induction and properly grounded at one location. The worker’s body, the transmission conductor and the grounding equipment make a conductive loop. If the worker positions themselves very close to the grounding equipment, the loop will be small and the generated voltage and current should also be low.
Magnetic field induction also occurs in underground cables when multiple sets of grounds are installed on a de-energized cable paralleling an energized cable carrying AC.
Magnetic field induction is possible in a de-energized and grounded conductor when one end of the de-energized conductor is laying on earth or a grounded surface, and the de-energized conductor parallels an energized carrying current line. This often occurs during storm work, reconductoring projects and new construction.
How Do We Reduce or Eliminate Electric and Magnetic Field Induction?
First, during your tailboard meeting, always discuss the potential of having or creating electric and magnetic field induction during the work procedure. Ensure you apply industry-accepted personal protective grounding procedures before working with de-energized conductors and cables. If the de-energized conductor to be worked does not parallel or cross an energized AC line, no electric or magnetic induction can be developed. If the de-energized conductor to be worked parallels or crosses an energized AC line, consider the possibility of electric field induction. If the de-energized conductor to be worked parallels an energized AC line carrying current, electric and magnetic field induction are possible.
Ground switches at the terminals of transmission lines can be used to reduce electric field induction effects during the installation and especially during removal of temporary protective grounding equipment. However, the use of ground switches and personal protective grounding equipment where the de-energized line parallels an energized and current-carrying line, can create electric and magnetic field induction. The development of electric and magnetic field induction can create potentially hazardous voltages and currents in the multigrounded, de-energized line. The potential hazards of developing magnetic field induction with the use of ground switches and personal protective grounding equipment should be clearly understood before any work begins. The use of ground switches can develop conductive loops and hazardous conditions for workers.
When an energized conductor or cable parallels a de-energized conductor, magnetic field induction effects in the de-energized conductor should be considered. Magnetic field induction will occur when the de-energized conductor is grounded at two or more locations along the length of the de-energized conductor. The voltage and current developed by magnetic field induction can be lethal if extreme caution is not taken. However, magnetic field induction hazards can be reduced by grounding the conductor or cable once, and working as close as possible to the personal protective grounding equipment.
Magnetic field induction can also occur when multiple crews, working the same line, ground at their work location, thus creating the conductive loop. If multiple crews must work on a de-energized line at the same time, and the possibility of developing magnetic field induction is present, extra precautions and a thorough discussion of the hazard with all involved should occur.
About the Author: Brian Erga, president of ESCI Inc., has more than 36 years of electric utility expertise and holds a BSEE degree. An expert on safety practices and work methods related to the electric utility industry, he is a member of IEEE/ESMO, NSC, NFPA and ASTM F18, and a member of NESC Subcommittee 8, responsible for NESC Part 4 “Rules for the Operation of Electric Lines.”